BIO 308 - Principles of Experimental
Embryology Dr. Daley
Environmental
Sex Determination
•
Echiuroid worm -
Bonellia
–
Baltzer (1914)
–
Female worm is rock
dwelling - 10 cm body & proboscis over a meter long
–
Male is 1-3 mm long
& lives inside the females uterus
& fertilizes her eggs
–
If larva lands on sea
floor becomes female
–
If larva lands on the
proboscis of a female - chemical attractants emitted
•
Larva will enter females
mouth, migrate to the uterus and differentiates into a male
Bonellia
Sex
Determination in Alligators
•
In many reptiles - e.g.
alligators and crocodiles - temperature determines sex
•
Egg temperature during
the 2nd and 3rd weeks of incubation is critical
•
Eggs incubated at
30° C or below produce females
•
Eggs incubated at
34° C or above produce males
•
Eggs incubated in nests
on levee’s (close to 34°C) - females
•
Eggs incubated in nests
in marshes (close to 30°C) - males
Adaptation
to Environment
•
Butterflies incubated at
different temps - different colors - called morphs
•
European Map Butterfly
–
Spring - bright orange & black spots
–
Summer form - mostly
black with white band
–
Controlled by day length
and temperature
Sunscreens
in Sea Urchins & Tunicates
•
Mycosporine compounds
give protection against UV-B radiation
•
Experimentally altering
the amount of mycosporine amino acids in sea urchin eggs showed that higher
levels gave more protection to embryos from UV damage
UV Repair
Enzymes in Frogs
•
The DNA repair enzyme,
photolyase excises and replaces damaged thymidine residues in amphibian eggs
and oocytes
•
Levels of this enzyme
varied 80 fold among tested species and correlated with the site of egg laying
•
Eggs exposed to the most
sun had highest levels of photolyase
–
Also correlated with
whether or not the species was suffering from population decline
Cell-to-Cell
Interactions in Development
•
Differentiation -
development of specialized cells
•
A cell must first commit
to a particular fate - called Commitment
–
Does not look different
but its developmental fate is restricted
•
Process of
commitment - two stages
–
Specification -
first stage - specified when it can differentiate autonomously in a neutral
environment - can still be reversed
–
Determination -
second stage - differentiate autonomously even when placed into another region
of the embryo - commitment is irreversible
Autonomous
Specification
•
Common in invertebrates
•
To demonstrate - a
blastomere is removed from an embryo -
•
In isolation it will
produce those cells it would have if left in the embryo
•
The embryo will be
missing those cells that would result from the blastomere - only those cells
•
Called mosaic
development - like a patchwork of independent self differentiating parts
Autonomous
Specification
•
Morphogenic determinates
(proteins or mRNA) are placed in different parts of the egg cytoplasm - divided
up as the embryo divides
– These specify cell type
•
In variant cleavages
mean the same lineages in each embryo - blastomere fates are invariant
•
Specification precedes
any large scale cell migration
Autonomous
Specification
Conditional
Specification
•
Hallmark is interactions
with neighboring cells
•
In all vertebrates and
some invertebrates
•
Each cell can
potentially become many different cell types
•
It is the interactions
with other cells that restricts the fate of one or both of the cells
•
Thus if a cell is
removed early in development the remaining blastomeres alter their fates to
compensate for the missing cells
Conditional
Specification
•
Conditional
specification brings about a pattern of development called regulative
development
•
This is critical in
identical twins
Conditional
Specification
Weismann’s
Germ Plasm Theory
•
Germ cells give rise to
the differentiating somatic cells & also new germ cells
•
Weismann postulated that
only germ cells contained all the inherited determinants
•
Somatic cells were each
thought to contain a subset of the determinants
•
Thus the determinants
found in the nucleus would determinate the cell type
Weismann’s
Germ Plasm Theory
Testing
Weismanns Theory of Inheritance
•
Defect Experiments
- one destroys a portion of the embryo - observe development of the impaired
embryo
•
Isolation experiments
- one removes a portion of the embryo - then observes the development of the
isolated part
•
Recombination
experiments - one observes the development of the embryo after replacing an
original part with a part from a different region
•
Transplantation
experiments - one portion of the embryo is replaced by a portion of a
different embryo - used in many early fate maps
Roux’s
Tests Weismann’s Hypothesis
•
Defect experiment - Got
half an embryo
•
Thus the frog
embryo - mosaic of self-differentiating
parts - just as Weismann had suggested
Driesch’s
Work on Regulative Development
• Did not fit Weismann’s and Roux’s
predictions!
Driesch’s
Pressure Plate Experiment
•
A recombination
experiment
Morphogen
Gradients
•
Cells fates can be
specified by soluble molecules secreted at a distance from the target cells
–
Called morphogens
–
May specify more than one cells by forming a
concentration gradient
•
Regeneration in flatworms
supports this idea
Syncytial
Specification
•
Common in insects
•
Interactions occur
between parts of cells
•
In these insects the
early cell divisions are not complete - nuclei divide within the egg cytoplasm
•
The egg thus contains
many nuclei - a syncytium
•
The cytoplasm is not
uniform - ant. Differs from posterior
•
Morphogens gradients control
the fate of the nuclei
•
One final note -
typically animals use more than one method of specification - Drosophila use a
all three we discussed
Syncytial
Specification in Drosophila
Morphogenesis
•
How are tissues formed
from populations of cells?
–
Layers of cells in the
retina
•
How are organs
constructed from tissues?
–
Eye - sclera, chroid
layer, lens and retina
•
How do organs form in
particular locations, and how do migrating cells reach their destinations
–
Eyes only develop in the
head, not the abdomen
–
Some cells like blood
cells and germ cells travel great distances to get to their final destinations
Morphogenesis
•
How do organs and their
cells grow, and how is their growth coordinated throughout development?
–
Neurons of the eye do
not divide after birth, however cells of the intestines are continually
sloughed off and need to be replaced on a daily basis - what controls the rate
of mitosis
•
How do organs achieve
polarity?
–
Look at a cross section
of any body part - tissues such as muscle, cartilage, bones, blood vessels etc.
are arranged differently in different part of the same body part such as an arm
or leg
Selective
Cell Affinity
•
Townes and Holtfreter
(1955)
–
Separated amphibian neurulae cells in alkaline solution
–
Mixture of cells
reaggregated spatially - epidermal cells moved peripherally, mesodermal cells
moved internally
–
Called this selective
affinity
•
Ectoderm - positive
affinity for mesoderm and a negative affinity for endoderm
•
Mesoderm has positive
affinity for both endoderm and ectoderm
Townes
and Holtfreter Experiment
Differential
Cell Affinity
•
Malcolm Steinberg
proposed a model that explained patterns of cell sorting based on thermodynamic
principles
•
Cells interact so as to
form an aggregate with the smallest interfacial free energy
•
Thus the cells rearrange
themselves into the most thermodynamically stable model
•
For this to work - only
need differences in the strengths of adhesion
–
This was demonstrated
Cadherins
and Cell Adhesion
•
Cadherins - major cell
adhesion molecule
–
Calcium dependent
adhesion molecules
•
These are anchored into
the cell by complex of proteins called catenins
•
The cadherin-catenin
complex forms the classic adherens junctions that connect epithelial cells
together
–
Catenins - bind to the
actin cytoskeleton
•
Cadherins join together
by binding to the same type of cadherin on another cell
–
Called homophilic binding
•
Cadherins molecules have
been shown to be essential for holding epidermis and neural tube cells together
during development
Cadherin-Mediated
Cell Adhesion
Localization
of Cadherins